1. Field of the Invention
The present invention relates generally to transmitters for radio communication systems and, in particular, to a high efficiency power control device capable of reducing noise, current consumption and the generation of spurious signals in a radio transmitter.
2. Description of the Related Art
In a radio communication system, a terminal apparatus and a base station apparatus include a power amplifier for amplifying a low power signal to a high power signal. A CDMA (Code Division Multiple Access) or PCS (Personal Communication System) phone deals with both very low power signals and very high power signals.
FIG. 1 is a block diagram illustrating a conventional transmitter for a terminal apparatus such as the CDMA or PCS phone. An IF (Intermediate Frequency) filter 110 receives a modulated IF input transmission signal S.sub.IF and filters the IF transmission signal. An IF amplifier 120 amplifies the IF transmission signal, and an IF filter 130 filters an output of IF amplifier 120. A mixer 140 mixes an output of IF filter 130 with a local oscillation frequency generated from a local oscillator 150 to generate an RF (Radio Frequency) signal. An RF filter 160 filters the mixed signal to detect the RF signal. A drive amplifier 170 amplifies the RF signal in order to obtain a power signal for driving a power amplifier 190. An RF filter 180 filters an output of drive amplifier 170. The power amplifier 190 amplifies the filtered RF signal. A duplexer 220 switches an antenna 230 to a transmitting part or a receiving part. An isolator 210, which is connected between an output node of power amplifier 190 and an input node of duplexer 220, transfers an output of power amplifier 190 to duplexer 220 without attenuation in order to prevent transmission distortion from a reflected wave. An automatic power controller (APC) 200 connected between drive amplifier 170 and power amplifier 190 automatically controls a gain of power amplifier 190.
It is to be appreciated that the above structure has numerous deficiencies, particularly with respect to a linear system having a large difference between the maximum power level and the minimum power level. For example, power amplifier 190 is always driven, irrespective of whether or not the transmission power is high or low. As a result, the current consumption of the transmitter is increased. Further, since the power amplifier has variable input/output characteristics according to an applied voltage, it is difficult to obtain optimal power amplifying efficiency when the applied voltage is changed by power control. Moreover, it is difficult to control a negative voltage of GaAs FETs (Field Effect Transistors) used for the power amplifier. Additionally, it is difficult to reduce noise and the generation of spurious signals in the transmitter.